Trivial insult son dismissed (5)
I believe the answer is:
light
'trivial' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for light that I've seen before include "weighing relatively little" , "Pane of glass" , "What smoker may be requesting" , "Illumination weights little" , "Opposite of dark" .)